Cryogenic grinding of tantalum for use in capacitor manufacture

ABSTRACT

An electrolytic capacitor comprising an anode comprised of cryogenically milled anode material is described. The cryogenic milling process prepares the active anode material for anode fabrication. The capacitor further comprises a casing of first and second casing members secured to each other to provide an enclosure. A feedthrough electrically insulated from the casing and from the casing and extending there from through a glass-to-metal seal, at least one anode electrically connected within the casing, a cathode, and an electrolyte. The cathode is of a cathode active material deposited on planar faces of the first and second casing members.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. patent applicationSer. No. 13/932,034, filed on Jul. 1, 2013, which claims priority fromU.S. Provisional Application Ser. No. 61/665,936, filed Jun. 29, 2012.

TECHNICAL FIELD

The present invention relates to the manufacture of capacitors, morespecifically, to the processing of materials that comprise the anode ofan electrolytic capacitor.

BACKGROUND OF THE INVENTION

As more and more medical applications are investigated and implementedto aid and assist the human body, devices needed to deliver the desiredtherapy are becoming increasingly more sophisticated, both functionallyand in terms of their structural makeup. Modern implantable devicesrequire power sources that are smaller in size, but powerful enough tomeet the therapy requirements. For example, a cardiac defibrillator hasa battery powering circuits performing such functions as, for example,the heart sensing and pacing functions. This requires electrical currentof about 1 microampere to about 100 milliamperes. From time-to-time, thecardiac defibrillator may require a generally high rate, pulse dischargeload component that occurs, for example, during charging of a capacitorassembly in the defibrillator for the purpose of delivering anelectrical shock to the heart to treat a tachyarrhythmia, the irregular,rapid heartbeats that can be fatal if left uncorrected. This requireselectrical current of about 1 ampere to about 4 amperes.

The current trend in medicine is to make cardiac defibrillators, andlike implantable devices, as small and lightweight as possible withoutcompromising their power. This, in turn, means that the componentswithin the capacitor, particularly the anode, need to be constructed tooptimum parameters as well as be free of contaminants.

Capacitor anodes typically comprise an anode active material such astantalum, aluminum, or niobium. The anode active material is generallymilled into a powdered form and pressed into a pellet. Furthermore, theanode material is generally sintered and then subjected to an anodizingor formation process before being incorporated into a capacitor. Ingeneral, the electrical performance of an electrolytic capacitor, suchas energy density and leakage current, can be improved by optimallycontrolling the particle size, morphology, oxidation state andcontamination level of the anode active material.

Current anode active material processing methods typically comprise alengthy multi-step process that is both cumbersome and time consuming.In addition, because of the many steps, the anode active materialresulting from these prior art material preparation processes isgenerally prone to process variability and the potential introduction ofcontamination which could degrade the electrical performance of theresulting capacitor.

One such prior art material preparation process is outlined in FIG. 4.This prior art anode active material preparation process consists ofapproximately eleven steps whereas the material preparation process ofthe current invention, outlined in FIG. 5, comprises only three steps.The simplified process of the present invention significantly reducesprocessing time and the potential for introducing contamination into theprocessed material. In addition, the simplified material preparationprocess of the present invention decreases the possibility ofintroducing error into the process. Furthermore, the anode materialpreparation process of the present invention improves materialconsistency, which as a result, improves the electrical performance ofthe capacitor.

What is needed, therefore, is a simplified, less cumbersome materialpreparation process that provides an anode active material with moreconsistent properties. In addition, what is needed is a simplifiedmaterial preparation process that is less prone to processing errors andthe potential of contaminating the material.

SUMMARY OF THE INVENTION

The present invention provides a process by which the anode activematerial, comprising the anode within an electrolytic capacitor, isprepared. More specifically, the present invention provides a simplifiedprocess by which the anode active material is prepared through cryogenicmilling prior to fabrication into an anode of an electrolytic capacitor.The material preparation process of the present invention prepares theanode active material with more consistency, proper oxidation state andreduced potential for contamination, thereby providing an electrolyticcapacitor with increased energy density and reduced current leakage.

In comparison, the prior art material preparation process shown in FIG.4 is a multi-step process that is both time consuming and cumbersome. Inaddition, the anode material produced by the prior art process issusceptible to possible compositional inconsistencies and contamination.The prior art anode material preparation process is a cumbersome processthat comprises as many as 13 steps. The parameters of each step arevariable and are largely dependent upon previously performed steps. Sucha complex process is not conducive to producing a material havingconsistent properties.

Electrolytic capacitors typically comprise anodes comprised of tantalum.At room temperature, tantalum is a ductile material that makes millingof the material difficult. Thus, in order for tantalum to be milled to adesired particle size, it is usually embrittled through the use of ahydrogen embrittlement process. In the process, the material is exposedto hydrogen gas at temperatures as high as 900° C. This embrittlementprocedure transforms the generally ductile tantalum material intobrittle tantalum that is easier for the material to be milled and brokendown into smaller particles. However, the hydrogen embrittlement processmodifies the material such that its surface properties, in particularits surface oxidation state is not ideal. Therefore, additionalprocessing steps are performed to adjust the properties of the anodeactive material to desired conditions.

Ideally the anode active material, specifically that of tantalum,comprises a layer of pentoxide along and within its surface. Theresistivity value of tantalum pentoxide is generally desirable forcontrolling current leakage in an electrolytic capacitor. However, ifdiffusion of oxygen is not correctly controlled, and too much oxygendiffuses into the surface of the tantalum, a less desirable phase oftantalum oxide is formed. Such undesirable phases of tantalum oxidetypically have a reduced electrical resistivity, which tends to increasethe current leakage.

If too much oxygen is dissolved into tantalum, the material is thengenerally subjected to a de-oxidation process whereby oxygen is removedthrough the introduction of magnesium into the material. The addition ofmagnesium, while desired to reduce the amount of oxygen from thetantalum surface, adversely contaminates the material. Thus, anadditional process whereby the magnesium is removed through an acidleaching process is typically performed. The acid leaching processutilizes caustic acids to remove the magnesium. However, this acidleaching process is not desirable because of the potential harm thecaustic acids may cause to both humans and the environment. In addition,the utilization of the acid leaching process requires additional acidremoval and containment procedures which add cost to the materialpreparation process.

Therefore, the material preparation process of the present invention wasdeveloped to address the shortfalls of the prior art process. Incontrast to embrittling the anode material through exposure in ahydrogen rich high temperature environment which adversely alters theoxidation state of the material, the present material preparationprocess utilizes cryogenic temperatures to freeze and embrittle theanode active material. Thus, by utilizing cryogenic temperatures, ofabout −150° C. or less, the anode material is frozen to an embrittledstate such that it can be effectively milled to a desired particle size.Such a milling process does not utilize hydrogen which adversely altersthe oxidation state of the material.

Specifically, at about −198° C., tantalum undergoes a phase change inwhich the ductile metal transitions into a body centered cubic (BCC)crystalline structure. At this temperature tantalum becomes embrittledwhich facilitates its milling without the need to expose it to gas, suchas hydrogen gas of the prior art. Thus, the potential to adversely alterthe surface oxidation state or phase of the material through exposure toa gas is reduced.

In contrast to the prior art, the anode active material, such astantalum, is milled in its frozen embrittled state. Since the hydrogenembrittlement and sequential de-hydride steps are eliminated, thesurface properties and electrical resistance of the anode material canbe better controlled. Furthermore, the need to subject the material toadditional, potentially environmentally harmful processing steps such asde-oxidation and acid leach is also eliminated.

Thus, the material preparation process of the present invention improvescontrol of the material properties of capacitor anode materials,particularly tantalum. In addition, the material preparation process ofthe present invention eliminates added processing steps, which thereforeincreases material preparation speed as well as reduces the potential ofintroducing contamination and error within the material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an embodiment of a capacitor of thepresent invention.

FIG. 2 illustrates a perspective view of an embodiment of an anodeassembly of the present invention.

FIG. 3 illustrates a cross-sectional view of an embodiment of acapacitor comprising one anode.

FIG. 3A illustrates a cross-sectional view of an embodiment of acapacitor comprising two anodes.

FIG. 4 is a flow chart illustrating the steps of the anode activematerial preparation process of the prior art.

FIG. 5 is a flow chart showing the steps of the anode active materialpreparation process of the present invention.

FIG. 6 illustrates an embodiment of a cryogenic milling instrument ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to the drawings, FIG. 1 is a perspective view of a capacitor10 according to the present invention. The capacitor 10 comprises atleast one anode composed of an anode active material and a cathode of acathode active material housed inside a hermetically sealed casing 12.The capacitor electrodes are operatively associated with each other by aworking electrolyte (not shown) contained inside the casing. The anodes,cathode and electrolyte of capacitor 10 will be described in detailhereinafter.

As particularly shown in FIGS. 1, 3 and 3A, the casing 12 is of metalmaterial comprising first and second casing members 14 and 16. Firstcasing member 14 comprises a first face wall 18 joined to a surroundingside wall 20 extending to an edge 22 (FIGS. 3 and 3A). Second casingmember 16 is in the shape of a plate and comprises a second face wall 24having a surrounding edge 26 (FIGS. 3 and 3A). The casing members 14 and16 are hermetically sealed together by welding the overlapping edges 22and 26 where they contact each other. The weld 27 is provided by anyconventional means; however, a preferred method is by laser welding.

A feedthrough 28 electrically insulates an anode terminal wire 30 fromthe casing 12. The terminal wire 30 extends from within the casing 12 tothe outside thereof. The location of a hole 32 in the surrounding sidewall 20 of the casing member 12 into which the feedthrough 28 is mountedis preferably offset towards the front edge 22 or towards the face wall18 in order to align with an embedded wire of one of the anodes.

Feedthrough 28 is preferably comprised of a glass to metal seal (GTMS)comprising a ferrule 34 defining an internal cylindrical through bore orpassage 36 of constant inside diameter. The ferrule 34 preferablycomprises a rectangular cross-section. However, the ferrule 34 may alsocomprise a cross-section of a cylindrical shape. The rectangularcross-sectional shape of the ferrule 34 provides a preferred surface onwhich the cathode current collector can be directly welded thereto.Furthermore, an insulative glass 40 provides a hermetic seal between thebore 36 and the anode terminal wire 30 passing therethrough.

The capacitor 10 further comprises at least one anode that isconnectable to the terminal wire 30 of feedthrough 28 within the casing12. FIG. 2 illustrates a perspective view of an embodiment of an anodeassembly 42 comprising a pair of anodes connected in parallel to theterminal wire 30. Although a parallel connection between anodes ispreferred, the anodes may be connected in an electrical series manner.

As shown in the embodiment of FIG. 2, the anode assembly 42 includes afirst anode pellet 44 and a second anode pellet 46. A first anode wire48A, having a first end portion 50, is shown embedded in the firstpellet 44. A second anode wire 48B, having a second end portion 52, isshown embedded in the second pellet 46. Anode wires 48A, 48B arepreferably electrically connected to an interior portion 54 of an anodeterminal wire 30 by a suitable joining process, such as laser welding,in a later manufacturing step. Alternatively, the anode wire may be of acontinuous length in which its end branches into two wires that are eachrespectively embedded into the first and second pellets 44, 46.Furthermore, as shown, a third anode wire 49 may be used to connect thetwo anode pellets 44, 46 in electrical series.

Each anode pellet 44, 46 is preferably composed of an anode activematerial 56 (FIG. 6). The anode active material of the anode pellets 44,46 is typically composed of a metal selected from the group consistingof tantalum, aluminum, titanium, niobium, zirconium, hafnium, tungsten,molybdenum, vanadium, silicon, germanium, and alloys and/or mixturesthereof in the form of a pellet. However, tantalum is a preferred anodematerial because its high dielectric constant facilitates the ability toproduce a capacitor having a reduced electrical current leakage.

An ideal tantalum anode comprises a highly porous structure having afairly uniform oxide surface thickness. Furthermore, it is preferredthat the surface of the tantalum material comprise a fairly uniformthickness of tantalum pentoxide. Tantalum pentoxide is preferred becauseof its increased electrical resistivity which tends to reduce electricalcurrent leakage within the capacitor.

Prior to preparing the anode active material 56 for fabrication intoanodes 44, 46, the initial or base tantalum material is generallycreated using one of two methods. In the first process, an ingot oftantalum is first subjected to hydrogen at elevated temperatures toembrittle the material. The embrittled tantalum is then crushed intopowder. In the second process, potassium tantalum fluoride is chemicallyreduced with sodium to produce the base tantalum powder. However, bothbase tantalum processes produce initial powders having varying particlesizes and morphologies. The varying range of particle sizes andmorphologies are not conducive for fabrication into an anode of anelectrolytic capacitor. Rather, a uniform particle size and morphologyis more conducive for electrolytic capacitor anode fabrication. Thecylindrically shaped tantalum particles having a more uniform particlesize diameter encourages the creation of a porous anode body.

The term “morphology” is herein defined as the physical appearance of amaterial such as the appearance when viewed with a scanning electronmicroscope. In a preferred embodiment, the particles of tantalum are ofa rod or cylindrically shape. Thus, the preferred morphology of theanode active material is one of a fibrous powder. The term “particlesize distribution” is defined herein as the distribution of particlesize with respect to cumulative percent of the material. For example, amaterial having a uniform particle size distribution may have a monomialparticle size distribution. A monomial particle size distribution occurswhen the majority of particles comprising the material have a singleparticle size.

The base tantalum material typically requires additional processing inwhich the material is made ready to be fabricated into an anode of anelectrolytic capacitor. Generally, these additional material processingprocedures modify the tantalum such that it has a more uniform oxidethickness, particle size and/or morphology. In a preferred embodiment,the tantalum comprises a layer of tantalum pentoxide. The properties oftantalum pentoxide are preferred for creating a capacitor having reducedcurrent leakage. Furthermore, the desired attributes of the processedanode active material facilitate creation of anodes having a relativelyhigh level of oxidized surface area and a dimensionally consistent coreof un-oxidized tantalum for conduction of electrical currenttherewithin.

FIG. 4 illustrates a flow chart of the tantalum material preparationprocess of the prior art that is used to prepare the anode activematerial 56 for incorporation into an anode. As illustrated, the processcomprises a time consuming and cumbersome eleven step process that isprone to possible processing inconsistencies. In addition, the materialpreparation process of the prior art is also prone to potentialcontamination.

As shown, the initial step of the prior art process involves hydrogenembrittlement of the base tantalum material. In this step, the tantalumis subjected to hydrogen gas at temperatures up to 900° C. for as longas 18 hours. The introduction of the hydrogen gas within the structureof the tantalum embrittles the material such that it can be more readilymilled. After the tantalum is embrittled, the material is thenmechanically milled.

After milling, the tantalum material is then subjected to a de-hydridingprocess in which the previously introduced hydrogen is removed from thestructure of the material. More specifically, the de-hydriding processsubjects the milled material to an increased temperature either in avacuum or argon rich environment to assist removal of hydrogen andmodification of the oxidation state. In an embodiment, a portion of thesurface thickness of the tantalum is oxidized into a more desirabletantalum pentoxide phase.

After being subjected to the de-hydriding step, the milled material isthen further subjected to an agglomeration process in which smallerparticles are combined to obtain a more desirable uniform or monomialparticle size distribution. In the agglomeration step, the milledtantalum powder is subjected to multiple cycles of a high temperatureenvironment of about 1,500-1,600° C. that join the particles together.

It is noted that in general, oxygen readily oxidizes the surface oftantalum. When the tantalum is prepared using the multi-step prior artprocess, the material undergoes multiple exposures of atmospheric oxygenfollowed by exposure to elevated temperatures. These cycles ofatmospheric oxygen and subsequent exposure to elevated temperatures,particularly at temperatures greater than 800° C., cause the surfaceoxygen to diffuse within the structure of the tantalum. Thus, given themultiple steps of the prior art process, control of the diffusedatmospheric oxygen within the material, and its resulting oxidationstate, is difficult to control.

A correct balance of material processing conditions is required toachieve the ideal tantalum pentoxide phase within the depth of thesurface of the tantalum. In particular, a correct amount of diffusedoxygen is required to achieve the ideal phase of tantalum oxide. If toomuch oxygen diffuses into the tantalum structure, an undesirable phaseof tantalum oxide could result having an electrical resistivity that istoo low for use in an electrolytic capacitor.

It is for this reason that a de-oxidation step may be required to removeaccess oxygen from the tantalum structure. More specifically, in thede-oxidation step magnesium is added to the tantalum material and themixture is further subjected to an increased temperature of about 800°C. to 1,000° C. An acid leach process comprising sulfuric acid and/orhydro fluoric acid is then utilized to remove magnesium from thetantalum. However, the acid leach step is not desirable because thecaustic liquid requires special handling and precautions. Furthermore,exposure to these acids is known to cause potential harm to humans andthe environment.

In contrast, the material preparation process of the present inventionis less cumbersome and eliminates many of these prior art processingsteps. Specifically, the anode active material preparation process ofthe present invention utilizes exposure to cryogenic temperatures,generally below −150° C., to embrittle the tantalum material formilling. Therefore, the need to adjust the oxidation state of the anodematerial through the de-hydriding and subsequent de-oxidation steps areeliminated.

In the material preparation process of the present invention, thetantalum material is subjected to a cryogenic liquid 57 such as liquidnitrogen, having a temperature of about −210° C. to about −195° C.(depending on atmospheric pressure), liquid helium having a liquidtemperature of about −269° C. or liquid hydrogen having a liquidtemperature of about −252° C. Once subjected to the cryogenictemperature, the tantalum material is then milled to a desired particlesize and distribution.

At ambient temperatures tantalum is generally a ductile material. Thus,when subjected to a mechanical stress, such as when subjected tomechanical milling, the material tends to bend rather than fracture.However, when subjected to cold temperatures, tantalum generally becomesbrittle and is therefore in a more ideal state for milling.Specifically, with regards to tantalum, the material undergoes a phasechange in which the ductile metal transitions into a crystalline bodycentered cubic crystal structure at about −198° C. Thus, subjectingtantalum to a cryogenic temperature of about −198° C., or less,transitions the material into a brittle crystalline form that is moreconducive to milling.

FIG. 5 further illustrates the flow of the material process of thepresent invention. As shown, after the anode active material is milledto a desired particle size, the powdered material may be subjected to apowder agglomeration step in which the milled anode material isagglomerated together to achieve a desired particle size distribution.Similar to the agglomeration step of the prior art, the cryogenicallymilled material may be subjected to an increased maximum temperature ofabout 1,600° C. to cause the milled material to agglomerate or bindtogether, thus adjusting the particle size distribution.

As previously mentioned, the simplicity and cryogenic temperaturesutilized in the material preparation process of the present invention,further reduce the potential for material contamination. In particular,it is believed that the material preparation process reducescontamination levels of hydrogen and iron. Specifically, it is believedthat the active anode material 56 produced by the present inventioncomprises less than 50 parts per million (ppm) hydrogen (H) and lessthan 10 parts per million (ppm) iron (Fe).

In a preferred embodiment, the resulting anode active material comprisesan average particle size diameter of less than about 5 μm. Morespecifically, the anode active material comprises an average particlesize diameter that ranges from about 0.5 μm to about 3 μm, morepreferably from about 1.25 μm to about 1.75 μm. The average particlelength of the anode active material ranges from about 5 μm to about 25μm, more preferably from about 10 μm to about 20 μm. The averageparticle length and diameter are preferably measured using a scanningelectron microscope in which a sample size of 30 is used to obtain theaverage values. In addition, the surface area of the final anode activematerial preferably ranges from about 0.3 m²/g to about 0.6 m²/g. Thesurface area is preferably measured using the Brunauer-Emmett-Tellersurface area measurement method. Furthermore, the finalized anodematerial should have a bulk density ranging from about 1 g/cc to about 3g/cc per ASTM specification B212.

FIG. 6 illustrates an embodiment of a milling instrument 58 used to millthe anode active material 56, such as tantalum, at a cryogenictemperature. As shown, the anode active material 56 is positioned withinthe mill instrument 58, such as an attritor mill. The mill 58 comprisesa bowl 60 in which the anode active material 56 is positioned. A seriesof mill blades 62 extend perpendicularly from a rotatable shaft 64 thatrotates in a clockwise and/or counter-clockwise manner within thematerial. Milling media 66 may also be added to the material 56 withinthe bowl 60 of the mill 58. The milling media may be of a spherical orcylindrical shape comprising zirconia, zirconia toughened alumina or ofthe material being milled such as tantalum. As further shown in FIG. 6,a jacket 68 may be positioned around the bottom of the bowl 60 of themill 58 such that a gap of space 70 resides between the external surfaceof the bowl and the interior surface of the jacket.

In an embodiment, a cryogenic liquid, such as liquid nitrogen or liquidhelium, may reside within this gap of space 70. Alternatively, thecryogenic liquid may be added directly to the anode active material 56within the milling instrument 58. The cryogenic liquid thus cools theanode active material 56 such that it becomes embrittled and easier tomill into small particles. The milling instrument 58 is not limited toan attritor mill. Other milling instruments and communition techniquessuch as ball milling, jet milling, vibratory milling, ultrasonic millingand hammer milling may also be used.

Once the anode active material is prepared, the powder is compressedinto a pellet having the previously described anode wires embeddedtherein and extending therefrom. The anode pellets 44, 46 are sinteredunder a vacuum at high temperatures. The porous pellets 44, 46 are thenanodized in a suitable electrolyte. This serves to form a continuousdielectric oxide film thereon. The anode assembly comprising the pellets44, 46 and their associated anode wires 48A, 48B, 49 is then formed to adesired voltage to produce an oxide layer over the sintered bodies andthe anode wires 48A, 48B, 49.

The capacitor 10 preferably comprises separators of electricallyinsulative material that completely surround and envelop the anodes. Forexample, the anode assembly 42 shown in the embodiment of FIGS. 3 and 3Acomprises a first separator 72 enclosing the first anode 44 and a secondseparator 74 enclosing the second anode 46. The separators 72, 74 may beformed as pouches that enclose the anode pellets 44 and 46. Inparticular, separator 72, 74 is sealed with a flap of material thatextends around the majority of the perimeter of anode pellet 44, 46. Theindividual sheets of separator material are preferably sealed by aprocess such as ultrasonic welding, or heat sealing.

Separators 72 and 74 prevent an internal electrical short circuitbetween the anode and cathode active materials in the assembledcapacitor and have a degree of porosity sufficient to allow flowtherethrough of the working electrolyte during the electrochemicalreaction of the capacitor 10. Illustrative separator materials includewoven and non-woven fabrics of polyolefinic fibers includingpolypropylene and polyethylene, or fluoropolymeric fibers includingpolyvinylidene fluoride, polyethylenetetrafluoroethylene, andpolyethylenechlorotrifluoroethylene laminated or superposed with apolyolefinic or fluoropolymeric microporous film, non-woven glass, glassfiber materials and ceramic materials.

Suitable microporous films include a polyethylene membrane commerciallyavailable under the designation SOLUPOR®, (DMS Solutech); apolytetrafluoroethylene membrane commercially available under thedesignation ZITEX®, (Chemplast Inc.) or EXCELLERATOR®, (W. L. Gore andAssociates); a polypropylene membrane commercially available under thedesignation CELGARD®, (Celgard LLC); and a membrane commerciallyavailable under the designation DEXIGLAS®, (C. H. Dexter, Div., DexterCorp.). Cellulose based separators also typically used in capacitors arecontemplated by the scope of the present invention. Depending on theelectrolyte used, the separator can be treated to improve itswettability, for example with a surfactant, as is well known by thoseskilled in the art.

The structure of the cathode is best understood with reference to FIGS.2, 3 and 3A. FIG. 3 illustrates a cross-sectional view of an embodimentof the capacitor 10 comprising a single anode 44. As shown in thefigure, a first anode 44 resides between portions of a cathode activematerial 76 that contacts the inner surfaces of the casing face walls 18and 24. In this embodiment, the capacitor 10 comprises a single anodesandwiched between layers of cathode active material 76 that arepositioned along respective inner surfaces of side walls 18 and 24.

Likewise, FIG. 3A illustrates an alternate embodiment in which thecapacitor 10 comprises a first and second anode 44, 46. As shown in FIG.3A, another portion of the cathode active material 76 is positionedintermediate the anodes 44 and 46. The cathode active material 76intermediate the anodes 44 and 46 is supported on opposed surfaces 78and 80 of a current collector 82 (FIGS. 2 and 3A), preferably in theform of a foil. It is further contemplated that the capacitor of thepresent invention is not limited solely to a single or dual anodestructure. For example, the capacitor of the present invention maycomprise three or more anodes connected in electrical series or parallelto the terminal wire 30. A more detailed discussion of embodiments ofthree anode capacitors may be found in U.S. Pat. No. 7,813,107 toDruding et al. and U.S. Pat. No. 7,983,022 to O'Connor et al., both ofwhich are assigned to the assignee of the present application and areincorporated by reference herein.

The cathode active material 76 preferably has a thickness of about a fewhundred Angstroms to about 0.1 millimeters directly coated on the innersurface of the face walls 18 and 24 of casing members 14 and 16, or itmay be coated on a conductive substrate (not shown) in electricalcontact with the inner surface of the face walls. In that respect, theface walls 18 and 24 and the current collector 82 may be of ananodized-etched conductive material, or have a sintered cathode activematerial with or without oxide contacted thereto, or be contacted with adouble layer capacitive material, for example a finely dividedcarbonaceous material such as graphite or carbon or platinum black, orbe contacted with a redox, pseudocapacitive or an under potentialmaterial, or an electroactive conducting polymer such as polyaniline,polypyrrole, polythiophene, and polyacetylene, and mixtures thereof.

According to one preferred aspect of the present invention, the redox orcathode active material includes an oxide of a first metal, the nitrideof the first metal, the carbon nitride of the first metal, and/or thecarbide of the first metal, the oxide, nitride, carbon nitride andcarbide having pseudocapacitive properties. The first metal ispreferably selected from the group consisting of ruthenium, cobalt,manganese, molybdenum, tungsten, tantalum, iron, niobium, iridium,titanium, zirconium, hafnium, rhodium, vanadium, osmium, palladium,platinum, nickel, and lead.

The cathode active material 76 may also include a second or more metals.The second metal is in the form of an oxide, a nitride, a carbon nitrideor carbide, and is not essential to the intended use of the conductiveface walls 18 and 24 and the intermediate current collector 82 as acapacitor electrode, and the like. The second metal is different thanthe first metal and is selected from one or more of the group consistingof tantalum, titanium, nickel, iridium, platinum, palladium, gold,silver, cobalt, molybdenum, ruthenium, manganese, tungsten, iron,zirconium, hafnium, rhodium, vanadium, osmium, and niobium. In apreferred embodiment of the invention, the cathode active materialincludes an oxide of ruthenium or oxides of ruthenium and tantalum.

The mating casing members 14 and 16, and the electrically connectedconductive substrate if it is provided, are preferably selected from thegroup consisting of tantalum, titanium, nickel, molybdenum, niobium,cobalt, stainless steel, tungsten, platinum, palladium, gold, silver,copper, chromium, vanadium, aluminum, zirconium, hafnium, zinc, iron,and mixtures and alloys thereof. Preferably, the face and side walls ofthe casing members 14 and 16 and the current collector 82 have athickness of about 0.001 to about 2 millimeters.

The exemplary electrolytic-type capacitor 10 shown in FIG. 3A has thecathode active material preferably coating the face walls 78 and 80,with the coating spaced from the side wall 24 of casing member 16 andthe peripheral edge of casing member 14. Such a coating is accomplishedby providing the conductive face walls 18 and 24 of the respectivecasing members 14, 16 with a masking material in a known manner so thatonly the intended area of the face walls is contacted with activematerial. The masking material is removed from the face walls prior tocapacitor fabrication. Preferably, the cathode active material issubstantially aligned in a face-to-face relationship with the majorfaces of the anodes 44 and 46.

The cathode current collector 82 may comprise a tab 84 extendingoutwardly therefrom. The tab 84 is not provided with active material.Instead, it is left uncovered. In a preferred embodiment, the tab 84 isdirectly connected to a planar face comprising the previously describedferrule 34 (FIG. 2).

When fabrication of the anode/cathode assembly is complete, it ispositioned inside the first casing member 14. The exposed distal portionof the feedthrough ferrule 34 is disposed in the opening 32 in side wall20 with the distal end of terminal wire 30 extending outside the firstcasing member. The exposed distal portion of the feedthrough ferrule 34is welded to side wall 20 to join and seal the feedthrough 28 to thecasing member 14.

Casing member 14 is then mated with casing member 16 and sealed thereto,preferably by laser welding. The outer edge 22 of casing member 14 isflush with side wall 24 at the outermost edge 26 of casing member 16,and a weld 27 is formed at the interface between the edges 22 and 26.For a more detailed discussion regarding various casing constructionssuitable for the present capacitor, reference is made to U.S. Pat. No.7,012,799 to Muffoletto et al. This patent is assigned to the assigneeof the present invention and incorporated herein by reference.

In a final step of providing capacitor 10, the void volume in casing 12is filled with a working electrolyte (not shown) through a fill opening86 (FIG. 1). This hole is then welded closed to complete the sealingprocess. A suitable working electrolyte for the capacitor 10 isdescribed in U.S. Pat. No. 6,219,222 to Shah et al., which includes amixed solvent of water and ethylene glycol having an ammonium saltdissolved therein. U.S. Pat. No. 6,687,117 to Liu and U.S. PatentApplication Pub. No. 2003/0090857 describe other electrolytes for thepresent capacitor 10. The electrolyte of the latter publicationcomprises water, a water-soluble inorganic and/or organic acid and/orsalt, and a water-soluble nitro-aromatic compound while the formerrelates to an electrolyte having de-ionized water, an organic solvent,isobutyric acid and a concentrated ammonium salt. These patents andpublications are assigned to the assignee of the present invention andincorporated herein by reference.

It is, therefore, apparent that there has been provided, in accordancewith the present invention, a capacitor containing at least one anodethat is connected to a common terminal within the capacitor casing.While this invention has been described in conjunction with preferredembodiments thereof, it is evident that many alternatives,modifications, and variations will be apparent to those skilled in theart. Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims.

What is claimed is:
 1. A method of fabricating a capacitor comprisingthe following steps: a) providing a case having first and second casingmembers; b) applying a cathode active material to a first face wall ofthe first casing member and a second face wall of the second casingmember; c) providing an anode active material comprising the followingsteps: i) providing a base anode material; ii) providing a millinginstrument; iii) positioning the base anode material within the millinginstrument; iv) cooling the base anode material to a temperature below−150° C.; and v) milling the base anode material to an average particlediameter of less than 5 mm; d) forming the anode active material into ananode and positioning the anode within the first casing member; e)securing the second casing member to the first casing member such thatthe cathode active material faces the anode within the case; and f)filling the case with an electrolyte.
 2. The method of claim 1 includingselecting the base anode material from the group consisting of tantalum,aluminum, titanium, niobium, zirconium, hafnium, tungsten, molybdenum,vanadium, silicon, germanium, alloys, and mixtures thereof.
 3. Themethod of claim 1 including milling the base anode material to anaverage particle diameter from about 0.5 μm to about 3 μm.
 4. The methodof claim 1 including milling the base anode material to an averageparticle length from about 5 μm to about 25 μm.
 5. The method of claim 1including milling the base anode material to an average particle surfacearea from about 0.3 m²/g to about 0.6 m²/g.
 6. The method of claim 1including milling the base anode material to a bulk density from about 1g/cc to about 3 g/cc per ASTM specification B212.
 7. The method of claim1 including selecting the milling instrument from the group consistingof an attritor mill, a jet mill, a vibratory mill, a ball mill, and ahammer mill.
 8. The method of claim 1 including adding milling mediawith the base anode material within the milling instrument prior to thecooling step.
 9. The method of claim 1 including providing a feedthroughhaving opposing proximal and distal terminal lead ends and positioningthe feedthrough within an opening of a sidewall of the casing such thatthe proximal terminal lead end resides within the case and the distalterminal lead end resides outside the case.
 10. The method of claim 9including electrically connecting the anode to the terminal leadproximal end.
 11. The method of claim 1 including selecting the cathodeactive material of an oxide, a nitride, or a carbon nitride from thegroup of metals consisting of ruthenium, cobalt, manganese, molybdenum,tungsten, tantalum, iron, niobium, iridium, titanium, zirconium,hafnium, rhodium, vanadium, osmium, palladium, platinum, nickel, lead,alloys, and mixtures thereof.
 12. A method of preparing an anode activematerial for incorporation within a capacitor, the method comprising thefollowing steps: a) providing a base anode material; b) providing amilling instrument; c) positioning the base anode material within themilling instrument; d) cooling the base anode material within themilling instrument to a temperature below −150° C.; and e) milling thebase anode material to an average particle diameter of less than 5 μm.13. The method of claim 12 including selecting the base anode materialfrom the group consisting of tantalum, aluminum, titanium, niobium,zirconium, hafnium, tungsten, molybdenum, vanadium, silicon, germanium,alloys, and mixtures thereof.
 14. The method of claim 12 includingmilling the base anode material to an average particle diameter fromabout 1 μm to about 2 μm.
 15. The method of claim 12 including millingthe base anode material to an average particle length from about 5 μm toabout 25 μm.
 16. The method of claim 12 including milling the base anodematerial to an average particle surface area from about 0.3 m²/g toabout 0.6 m²/g.
 17. The method of claim 12 including milling the baseanode material to a bulk density from about 1 g/cc to about 3 g/cc perASTM specification B212.
 18. The method of claim 12 including selectingthe milling instrument from the group consisting of an attritor mill, ajet mill, a vibratory mill, a ball mill, and a hammer mill.
 19. Themethod of claim 12 including adding milling media with the base anodematerial within the milling instrument prior to the cooling step. 20.The method of claim 12 including fabricating the anode active materialinto a capacitor anode.